Conventional simple telephone dialling keypad means generally comprise twelve keys or buttons to which ten digits 0-9 and special characters "*" and "#" are assigned. However, in more complex systems also a number of letters and other special characters such as punctuation symbols may be assigned to each key. In these extended keypad means, each key normally represents a group of three or four characters. Extended telephone dialling keypad means are used, inter alia, for storage and retrieval of data representative of names including telephone numbers of subscribers of a telephone system, e.g. a private branch network. Selection of a particular character of a group is established by actuating the corresponding key in a prescribed manner.
European patent application 0,608,682 discloses a keypad means having separate keys for designating the position of a character in a group. To select a character from a group, two keys have to be actuated: i.e. the key to which the relevant group of characters is assigned and a key designating the position of a character in that group.
British patent application 2,128,384 discloses a method for the selection of characters from a group of characters assigned to the keys of an extended telephone keypad by multiple actuation of a key, such that one key pressing represents a first character of the group, two key pressings result in the selection of a second character of the group etc. To discriminate between subsequent key pressings for the selection of a character of a group and the completion of the selection itself, a time delay interval is started each time after the deactivation of a key, i.e. the pressing thereof.
Activation of the key within this time delay contributes to the selection of a character of the group associated with the particular key. Lapse of the time delay interval following the last key pressing will result in displaying the selected character and making the keyboard ready for further inputs.
Although this method of selecting characters from a group of characters assigned to a single key of a keypad means already limits the number of keys which have to be actuated for forming a string of characters, such that no other keys have to be actuated than those to which the particular characters have been assigned, it has been found that this known method is not very suitable in accessing data entries such as telephone system subscriber data stored in a memory of a telephone set, for example. This, because typing errors, which can very easily occur with unexperienced users of such an extended telephone keypad means, may result in a string not matching any data entry stored in the memory. However, in these cases, the user will be generally notified of his typing errors only after he has completed the search string when he recognizes that there is no matching entry in the memory at all. Accordingly, the user has to start a new selection or has to use an erase key or the like, in order to correct his entry. It will be appreciated that this may not only take a relatively long time before a call can be placed, but will also get very annoying.